Evolutionary Biology Terms Starting With G
Evolutionary Biology Glossary: G
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Genetic Bottleneck
/ juh-NET-ik BOT-ul-nek / · Greek genesis meaning origin and bottleneck as a narrowing metaphor
Genetic bottleneck is a sharp, chance-driven reduction in population size that eliminates many alleles from the gene pool, leaving survivors with far less genetic variation than the original population carried.
When a population crashes to a small number of individuals, most alleles present at low frequency disappear simply because no carrier survives, regardless of whether those alleles were harmful or beneficial. Recovery in population size does not restore lost alleles, because new copies can only arise through mutation, which is far too slow to replace the diversity lost in a single crash. Reduced variation increases the frequency of inbreeding, which can expose recessive deleterious alleles and raise susceptibility to disease.
Northern elephant seals (Mirounga angustirostris) were hunted to an estimated 20 to 30 individuals by the 1890s; genetic surveys of modern populations numbering over 200,000 animals show extremely low heterozygosity compared with their southern relatives, which were never bottlenecked as severely.
Cheetahs (Acinonyx jubatus) experienced at least one severe bottleneck roughly 10,000 to 12,000 years ago, leaving the species so genetically uniform that skin grafts between unrelated individuals are rarely rejected, a result that would be impossible in most mammals with normal levels of genetic diversity.
A population quickly regains all lost genetic diversity after its numbers recover. Population size can rebound within decades, but alleles lost during the crash are gone permanently unless reintroduced by migration or recreated by new mutation.
Northern elephant seals were reduced to roughly 20 to 30 individuals along the Baja California coast in the late nineteenth century. Modern populations exceed 200,000 animals yet carry some of the lowest genetic diversity recorded in any mammal, with many gene loci showing near-complete uniformity across the entire species.
Elephant →Gradualism
/ GRAJ-oo-uh-liz-um / · Latin gradus meaning step
Gradualism is the hypothesis that evolutionary change accumulates through many small, incremental steps spread across long periods of geological time, rather than through sudden jumps.
Evolutionary gradualism proposes that allele frequencies shift continuously across generations as natural selection and genetic drift favor small heritable variations, producing detectable anatomical change only after thousands to millions of generations. Fossil series from planktonic foraminifera, tracked through deep-sea sediment cores spanning tens of millions of years, show smooth, measurable transitions in shell size and chamber shape that match gradualist predictions. Charles Darwin championed this view in “On the Origin of Species” (1859), partly because he believed the fossil record’s apparent gaps reflected incomplete sampling rather than genuine stasis.
Gradualism contrasts with punctuated equilibrium, proposed by Niles Eldredge and Stephen Jay Gould in 1972, which holds that most morphological change occurs rapidly during speciation events and that long periods of stasis dominate the record between those bursts.
Stickleback fish (Gasterosteus aculeatus) populations colonizing freshwater lakes after the last ice age have lost bony armor plates within as few as 50 generations, demonstrating that gradualist accumulation of small changes can proceed far faster than the geological timescale Darwin envisioned.
Gradualism says evolution never speeds up or slows down. Gradualism requires only that change accumulates through many small steps; the rate of those steps can vary enormously across lineages and environments.
Planktonic foraminifera preserved in Caribbean deep-sea sediment cores show gradual shifts in shell coiling direction and chamber size across roughly 10 million years of strata. Researchers measure these changes in fractions of a millimeter per million years, providing some of the clearest quantitative support for gradualist change in the fossil record.
Group Selection
/ GROOP sih-LEK-shun / · Old French groupe meaning cluster and Latin selectio meaning choosing
Group selection is a proposed mechanism of natural selection in which heritable traits spread because groups bearing those traits survive and reproduce more successfully than competing groups, even when the traits reduce the fitness of individual carriers within those groups.
The concept gained prominence through V.C. Wynne-Edwards’s 1962 argument that animals restrain their own reproduction to prevent overpopulation, a claim that triggered decades of debate. Most evolutionary biologists concluded that individual-level selection and kin selection, formalized through W.D.
Hamilton’s inclusive fitness framework in 1964, explain the same cooperative behaviors without invoking group-level competition. Multilevel selection theory, developed by David Sloan Wilson and others from the 1970s onward, attempts to reconcile these views by treating group selection and individual selection as simultaneous processes operating at different levels.
Experimental evolution studies with flour beetles (Tribolium castaneum) in the 1970s by Michael Wade provided some of the clearest laboratory evidence that selection among groups can produce trait changes that individual selection alone would not predict, helping revive serious scientific interest in multilevel selection.
Animals routinely sacrifice themselves for the good of their species. Evolutionary explanations focus on effects on individual reproduction and genetic relatives; sacrifice for the abstract benefit of a species, rather than for identifiable kin or group members, lacks a demonstrated mechanism.
Honeybee (Apis mellifera) colonies compete as units when swarms establish new nests, and colonies with more effective foraging and defense behaviors leave more daughter swarms per season. Researchers studying colony-level heritability of these traits use them to test whether selection among colonies can drive trait evolution independently of within-colony individual selection.